1. Field of the Invention
The present invention relates to a surface acoustic wave device especially having a balance-unbalance conversion function.
2. Description of the Related Art
Remarkable technical progress has been made in recent developments of small-sized and light-weight portable telephones. In order to produce such portable telephones, parts with a combination of plural functions have been made as well as the number of components has been reduced and the size of the components has been decreased. In these situations, in recent years, an extensive investigation of surface acoustic wave filters for use in the RF stages of portable telephones, having a balance-unbalance conversion function, a so-called balun function, had been made, and such surface acoustic wave filters have been used mainly in the GSM system (Global System for Mobile Communications).
In the case in which a balanced line such as a twin-lead type feeder and an unbalanced line such as a coaxial cable are connected directly to each other, unbalanced current flows, and undesirably, the feeder itself functions as an antenna. Thus, the team xe2x80x9cbalunxe2x80x9d means a circuit for preventing the unbalanced current and matching the balanced line and the unbalanced line to each other.
Referring to conversion between a balanced signal and an unbalanced signal, in some cases, the input impedance (e.g., 75xcexa9 for an unbalanced signal) and the output impedance (e.g., 300xcexa9 for a balanced signal) are different from each other by approximately a factor of four.
As the basic configuration of a surface acoustic wave device having the above-described balance-unbalance conversion function, the configuration shown in FIG. 14 is widely used. Longitudinally coupled resonator type surface acoustic wave elements 101 and 102 are contained in the configuration shown in FIG. 14.
The surface acoustic wave element 101 contains interdigital electrode portions referred to as an interdigital transducer, hereinafter referred to as IDT, 104, 103, and 105, and has reflectors 106 and 107 arranged so as to sandwich the IDTs.
The surface acoustic wave element 102 contains three IDTs 109, 108, and 110 arranged along the propagation direction of a surface acoustic wave and, moreover, reflectors 111 and 112 arranged so as to sandwich these IDTs.
The above-described configuration contains an unbalanced terminal 113 which causes the terminals on one side of the surface acoustic wave elements 101 and 102 to be electrically connected in parallel to each other, and balanced terminals 114 and 115 which are connected in series with the terminals on the other side of the surface acoustic wave elements 101 and 102.
The surface acoustic wave element 101 and the surface acoustic wave element 102 are different from each other in that the IDT 103 and the IDT 108 are inverted with respect to each other, and thereby, signals output from the balanced terminal 114 and the balanced terminals 115 are 180xc2x0 out-of-phase. Thus, an unbalanced signal input through the unbalanced terminal 113 is converted to balanced signals which are output from the balanced terminals 114 and 115.
In the above-described configuration, the output impedance is about four times of the input impedance. By exchanging the input terminal and the output terminals in the configuration of FIG. 14, a surface acoustic wave device in which the input impedance is about four times of the output impedance, and balanced inputs and an unbalanced output signal are applied can be provided.
Furthermore, for the surface acoustic wave device having a balance-unbalance conversion function, an excellent attenuation characteristic presented out of the pass band is required. Japanese Unexamined Patent Application Publication No. 6-177697 discloses a method of improving the above-described out-band characteristics. In this Patent Application, as shown in FIG. 15, the configuration is used in which surface acoustic wave resonators 202 and 203 are electrically connected in series with the input and output terminal sides of the surface acoustic wave element 201, respectively.
FIG. 16 shows an example of a surface acoustic wave device having a balance-unbalance conversion function which adopts the above-described configuration. Surface acoustic wave resonators 303 and 304 are connected in series with the input sides of the surface acoustic wave elements 301 and 302, respectively, and surface acoustic wave elements 305 and 306 are connected in series with the output sides, respectively. With this configuration, high attenuation and balance-unbalance conversion can be realized.
Referring to the transmission characteristics in the pass bands caused between the unbalanced signal terminal and the balanced signal terminals of the surface acoustic wave device having a balance-unbalance conversion function, it is required that the amplitude characteristics should be the same, and the phases should be reversed from each other by 180xc2x0. These characteristics are expressed in terms of an amplitude balancing degree and a phase balancing degree.
Referring to the amplitude and phase balancing degrees, the surface acoustic wave device having a balance-unbalance conversion function is assumed as a three port device, and for example, the unbalance input terminal is referred to as a first port, and the balanced output terminals are referred to as second and third ports, respectively. In this case, the amplitude balancing degree [A] is defined by A=[20log(S21)]xe2x88x92[20log(S31)], and the phase balancing degree [xe2x88x92180] is defined by B=[ less than S21xe2x88x92 less than S31], in which S21 represents a transfer coefficient for the transfer from the first port to the second port, and S31 represents a transfer coefficient for the transfer from the first port to the third port. The symbol [ ] represents the absolute value. Regarding the balancing degrees, the amplitude balancing degree is zero dB, and the phase balancing degree in the pass band of a surface acoustic wave device is zero degree in the ideal condition.
However, in practice, deviations from the balancing degrees are caused in the configuration of FIG. 16. The values are on such a large level as to become problems in practical use. The reason lies in that in the configuration of FIG. 16, the electrode fingers of the IDT 307 adjacent to the IDTs 308 and 309 are grounding electrode fingers, while the electrode fingers of the IDT 312 adjacent to the IDTs 313 and 314, respectively, are signal electrode fingers.
In the case in which the signal electrode and the grounding electrode are adjacent to each other in each of the IDT-IDT interval areas, the efficiency of conversion to electric current caused in the resonance mode having an intensity peak in each IDT-IDT interval area is improved. Thus, the insertion loss in the pass band, especially on the high frequency side, is decreased compared with the case in which the grounding electrode and the signal electrode are adjacent to each other. Moreover, the pass bandwidth is increased, and also, a deviation is caused in the phase-relationship. FIG. 17 shows differences between the frequency characteristics of the surface acoustic wave filters 320 and 321 shown in FIG. 16 and those between the phase characteristics thereof (the matching is made at 100xcexa9, and the characteristics are obtained in the configuration of FIG. 16).
The difference between the frequency characteristics of the surface acoustic wave filters 320 and 321 becomes large especially on the high frequency side of the pass band. Moreover, the phase characteristics of the surface acoustic wave filters 320 and 321 are not completely inverted from each other, and some deviation from the complete inversion is present. If a surface acoustic wave device is formed using the surface acoustic wave filters 320, and 321, these differences will deteriorate the balancing degrees.
To solve the above-described problems, preferred embodiments of the present invention provide a surface acoustic wave device having a balance-unbalance conversion function which includes first and second longitudinally coupled resonator type surface acoustic wave elements each having three interdigital electrode portions arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, the second surface acoustic wave element having a phase-relationship between the center interdigital electrode portion of the above-described interdigital electrode portions and the interdigital electrode portions sandwiching the center electrode portion which is inverted with respect to the phase-relationship of the first surface acoustic wave element, an unbalanced terminal provided in which each terminal on one side of the first and second surface acoustic wave elements is electrically connected in parallel to each other, and balanced terminals to which each terminal on the other side of the first and second surface acoustic wave elements is electrically connected in series, and an electrode finger pitch in the interdigital electrode portions of the first acoustic wave element is different from the electrode finger pitch in the interdigital electrode portions of the second acoustic wave element.
According to the above-described configuration, the balancing degrees caused between the balanced terminals is greatly improved, since the pitch in the IDTs of the first surface acoustic wave element is different from that in the IDTs of the second surface acoustic wave device.
Preferably, the value of xcexir/xcexis is in the range between about 0.9982 or greater and less than about 1, in which xcexis represents the electrode finger pitch in the IDTs of the first surface acoustic wave element, and xcexir represents the electrode finger pitch in the IDTs of the second surface acoustic wave element. The value of fis/fir may be in the range between about 0.9982 or greater and less than about 1, in which fis represents the frequency caused in the first surface acoustic wave element, and fir represents the frequency caused in the second surface acoustic wave element.
According to the above-described configuration, the improvement of the balancing degrees between the balanced terminals are secured, since the value of xcexir/xcexis or the value of fis/fir is set to be in the range between about 0.9982 or greater and less than about 1.
The frequency fis of the first surface acoustic wave element depends on the piezoelectric substrate, the sound velocity determined by the electrode occupied area ratio in the IDTs of the first surface acoustic wave element disposed on the substrate and the film thickness, and the electrode finger pitch of the EDTs.
The frequency fir of the second surface acoustic wave element depends on the piezoelectric substrate, the sound velocity determined by the electrode occupied area ratio in the IDTs of the second surface acoustic wave element disposed on the substrate and the film thickness, and the electrode finger pitch of the IDTs.
The frequencies fis and fir are preferably set to be different from each other, using the electrode finger pitches of the IDTs. However, the frequencies fis and fir may be set to be different from each other, using another configuration in which the electrode finger pitch of the IDTs is irregular in the propagation direction of a surface acoustic wave, e.g., the pitch of one pair of electrode fingers in an IDT is set to be different from the electrode finger pitch of the other IDTs, and moreover, the ratio of the frequencies caused in the first and second surface acoustic wave elements is set to conform to the above-described conditions so that a desired frequency characteristic can be attained.
According to another preferred embodiment of the present invention, a surface acoustic wave device having a balance-unbalance conversion function includes first and second longitudinally coupled resonator type surface acoustic wave elements each having a plurality of interdigital electrode portions arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave, the second surface acoustic wave element having a phase-relationship between the center interdigital electrode portion of the above-described interdigital electrode portions and the interdigital electrode portions sandwiching the center electrode portion which is inverted with respect to the phase-relationship of the first surface acoustic wave element, first and second one-terminal-pair surface acoustic wave resonators electrically connected in series to at least one terminal of the first and second surface acoustic wave elements, respectively, whereby first and second surface acoustic wave filters are provided, an unbalanced terminal causing the terminals on one side of the first and second surface acoustic wave filters to be electrically connected in parallel to each other, and balanced terminals electrically connected in series with the terminals on the other side of the first and second surface acoustic wave filters, respectively, and electrode finger pitch of the first one-terminal-pair surface acoustic wave resonator is different from the electrode finger pitch of the second one-terminal-pair surface acoustic wave resonator.
According to the above-described configuration, the balancing degrees caused between the balanced terminals is greatly improved, since the electrode finger pitch in the IDT of the first one-terminal-pair surface acoustic wave resonator is different from that in the IDT of the second one-terminal-pair surface acoustic wave resonator.
Preferably, the value of xcextr/xcexts is in the range between about 0.994 or greater and less than about 1, in which %ts represents the electrode finger pitch in the IDT of the first one-terminal pair surface acoustic wave resonator, and xcextr represents the electrode finger pitch in the IDT of the second one-terminal-pair surface acoustic wave resonator. The value of fis/fir may be in the range between about 0.994 or greater and less than about 1, in which fts represents the frequency caused in the first one-terminal-pair surface acoustic wave resonator, and ftr represents the frequency caused in the second one-terminal-pair surface acoustic wave resonator.
According to the above-described configuration, the improvement of the balancing degrees between the balanced terminals are secured, since the value of xcextr/xcexts or the value of fts/ftr is in the range between about 0.994 or greater and less than about 1.
The frequency fts of the first one-terminal-pair surface acoustic wave resonator depends on the piezoelectric substrate, the sound velocity determined by the electrode occupied area ratio in the IDT of the one-terminal-pair first surface acoustic wave resonator disposed on the substrate and the film thickness, and the electrode finger pitch in the IDT.
The frequency ftr of the second one-terminal-pair surface acoustic wave resonator depends on the piezoelectric substrate, the sound velocity determined by the electrode occupied area ratio in the IDT of the second one-terminal-pair surface acoustic wave resonator disposed on the substrate and the film thickness, and the electrode finger pitch in the IDT. In the above-description, the frequencies fts and ftr may be set to be different from each other, using the electrode finger pitches of the IDTs. However, the frequencies fts and ftr may be set to be different from each other, using another configuration in which the electrode finger pitch in the IDT is set to be irregular in the propagation direction of a surface acoustic wave, e.g., the pitch of one pair of electrode fingers in the IDT is set to be different from the electrode finger pitch of the other IDT, and moreover, the ratio of the frequencies caused in the first and second one-terminal-pair surface acoustic wave resonators is set to conform to the above-described conditions so that a desired frequency characteristic can be attained.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.